Loud CPU fan in my dad's old Dell Optiplex GX260 machine.

Does anyone have any ideas why my father's old Dell Optiplex GX260 machine's CPU fan is loud in blowing (not loud as spinning)? We opened it up and found out it is
from the CPU fan (the one with green plastic cover. We noticed the
interior and fans don't look that dusty/dirty (amazed at that compared
to my PCs after all these years).

I checked the CMOS to see if I could get fan readings or control, but
didn't see one.

Any ideas? Do we need to replace it? Both PSU and CPU fans seem to be
blowing fine. Just CPU fan is LOUD and blowing hard. And the room
temperature isn't even hot which is weird! I wonder if this is related
to the low voltage battery error in CMOS a few months ago (haven't seen
it since then).

I told him not to take the CPU fan parts apart because he might make it
worse. ;)

Yes, the fan sounds like an airplane. It reminds me of those modders who have fans cranking at high speed and so loud. Or being in server rooms that are loud! Definitely not fan noises. Just loud.

Motherboard capacitor failure: How long will the PC last without problems? So far so no problems. Just loud. And how do we fix it? Replace the whole motherboard? It's old and I don't think Dell has replacements?

A status update since the CPU fan just stopped a few minutes ago, but the PC is still working (no reboots, no lockups, no crashes, etc. yet!).

Not sure if the CPU fan is dead though. PSU fan is still spinning, black CPU fan is not spinning at all, but we can see it shaking/vibrating (not sure if it is from it or the PSU fan -- can feel air flows from both vents). Not sure if it is trying to spin or just dead. Computer still work. I did warn him to save his work and copy it to his flash drive in case computer overheats or whatever. It's like 80F degrees in the room too due to the current heat wave.

And, the fan spins at full speed again for who knows how long after
powering on for today! I did tell my father to not to use the PC when
the CPU fan stops again like last night to avoid heat damages to the old
CPU.

I hope to get the fan and/or motherboard replacement this long weekend.

The issue has been resolved. A computer hardware guy/friend came to fix
it since I am not a handy, due to my multiple disabilities. He said it
was the CPU fan with its broken sensors (not physically) after replacing
the CPU fan with a new $11 transparent Antec fan. It doesn't have a
sensor (don't need it) and very quiet (can't hear it compared to mine).

Weird. Earlier, it said out of range for CPU fan in BIOS (stopped to warn us) and CMOS event logs. Eh, what the heck is that? Is that normal when changing the fan? It is not the Dell's fan we're using too. It is a generic fan hacked into the Dell CPU fan container (whatever you call it).

PC still seems to work so far. Weird is that my friend and I didn't see this last Saturday when we rebooted a few times. We even turned off and on the PC a couple times too.

I also noticed my computer clock was wrong because the battery was swapped earlier with the fan replacement.

Ugh, and now a Dell program pop-up in Windows 2000 SP4 mentions it too! Argh! Does anyone know what controls it? I don't see a Dell program listed in Add/Remove Programs and services list.

I think it is the BIOS that monitors the fan speed, so I'd recommend using a system fan with the correct speed-monitoring hardware.

Once, I was able to use a generic unit for such duties with a Dimension 4550 by swapping the connector from a broken fan to a new, generic one (using a pin to pry back the tangs holding in the leads). Dell's fan connector was different that that used by standard third-party units.

Right, but BIOS tells the Dell software to pop-up to tell me that in Windows 2000 SP4. Windows, itself, wouldn't do that unless there is a software to check Dell BIOS. I am looking for that software to disable/remove. I *KNOW* the readings are incorrect. I just need to make it shush.

And I also noticed the computer's date got resetted back to 2004 twice so far. Ugh, I think the replaced CMOS battery is bad or something. Sheesh, this darn old PC!